Commercial hydronic heating systems are used in buildings to supply heat via hot water that is circulated by a pump. One type of traditionally common heating system utilizes non-condensing boilers in which a portion of the input heat is lost via the boiler flue. A more modern type of heating system increases overall efficiency by employing condensing boilers. A condensing boiler recovers a large amount of heat from exhaust gases that would otherwise be lost via the flue. Through the use of heat exchangers, the temperature of the gases exiting the flue may be reduced from a range of 300° F.-450° F. to a range of 90° F.-130° F.
Although it is becoming more of a standard practice to employ hydronic heating systems that utilize high-efficiency condensing boilers, many existing heating systems employ one or more non-condensing boilers of an age or condition that does not justify replacement of those boilers from an economic standpoint, as associated radiators, heat exchangers, or other end user heat transfer or transmission devices would require replacement as well. The result is that many buildings may have both non-condensing as well as condensing boilers, thereby forming what is known as hybrid heating systems. Although the overall efficiency of a condensing boiler system is higher than a non-condensing system, each type of boiler has certain advantages over the other depending upon factors such as outside air temperature and the heat load on the boiler. The present disclosure is directed to an arrangement for controlling a hybrid heating system in which each type of boiler can be realized to its particular advantage.